In order to manufacture reliable gallium nitride (GaN) based high electron mobility transistor (HEMT) devices and circuits, the amount of residual stress in a device's epitaxial structure must be controlled such that the effects of inverse piezoelectric stresses and/or stress enhanced corrosion of barrier layers is mitigated. Several methods of stress control have been investigated. Some of these methods include replacing barrier layers with indium aluminum nitride (InAlN) or indium gallium aluminum nitride (InGaAlN) lattice matched alloys. Other methods focus on lattice engineering the buffer layer with low aluminum concentration aluminum gallium nitride (AlGaN), AlGaN/GaN superlattices or dilute boron gallium nitride (BGaN) alloys. However, such approaches in reducing lattice stress have certain limitations.
Another issue pertains to maintaining high sheet charge density within a HEMT device while providing lattice stress relief. The high sheet charge density and high breakdown field of GaN based field effect transistors (FETs) enables significantly higher power operation compared to traditional group III-V HFETs. Commercially available GaN FETs have a sheet charge density <1×1013 cm−2. However, the highest charge density available for a GaN based materials system is greater than 5×1013 cm−2, which has been demonstrated with an AlN/GaN HFET. In such a system, the AlN is fully strained. However, the AlN barrier layer can readily form micro cracks if the thickness of the AlN barrier layer is beyond the critical thickness for lattice relaxation. In fact, even with an AlN thickness of approximately 4 nm, micro cracks would develop over time. Though the high sheet charge in an AlN/GaN system is attractive, the limitation of the strain makes a FET or HEMT device application impractical. To reduce strain energy, the thickness of the AlN barrier layer is reduced, which in turn dramatically reduces the sheet charge density. As such, an AlN system is far less attractive in comparison to a conventional AlGaN/GaN system.
One approach to minimizing total strain is the use of a lattice matched InAlN/GaN system. The sheet charge in a lattice matched condition is approximately 2.6×1013 cm−2. However, the poor crystal quality of InAlN, due mainly to phase segregation, limits applications for FETs and HEMTs. Furthermore, in addition to a very narrow growth window for closely lattice matched InAlN/GaN, sheet charge density decreases dramatically when the barrier is slightly compressively strained. As such, a need remains for providing lattice stress relief while maintaining high sheet charge density within a HEMT device.